Flexible unbonded pipes of the present type are for example described in the standard “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and the standard “Specification for Unbonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008. Such pipes usually comprise an innermost sealing sheath—often referred to as an internal pressure sheath, which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or usually a plurality of armoring layers. Often the pipe further comprises an outer protection layer which provides mechanical protection of the armor layers. The outer protection layer may be a sealing layer sealing against ingress of sea water. In certain unbonded flexible pipes one or more intermediate sealing layers are arranged between armor layers.
In general flexible pipes are expected to have a lifetime of 20 years in operation.
The term “unbonded” means in this text that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armoring layers located outside the inner sealing sheath and optionally an armor structure located inside the inner sealing sheath normally referred to as a carcass.
These armoring layers comprise or consist of multiple elongated armoring elements that are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
Unbonded flexible pipes are often used e.g. as riser pipes in the production of oil or other subsea applications. One of the difficulties in the production of crude oil and other fluids from reserves located in deep waters is that the crude oil normally has a temperature which is relatively high compared to the temperature of the surrounding sea water and during transportation from the reservoir to a top-site production platform or when transported in a flow line, the fluid is cooled down to a lower temperature which may increase the viscosity of the fluid or even result in more or less blocking of the pipe due to the formation of hydrates and waxes or other solidified substances.
In order to avoid undesired cooling down of a fluid in an unbonded flexible pipe, it is well known to provide the unbonded flexible pipe with one or more thermal insulation layers. The thermal insulation of subsea pipelines is a practice which in certain situations does not provide a sufficient protection against formation of solidified substances in the fluid, such as in case of temporary production stop. As temporary production stops cannot be fully avoided, it is essential that the pipe system is designed to ensure that the pipe is not blocked by solidified substances during a temporary production stop. Removal of a blocking in a pipe can be very difficult and costly and in worst case it is not possible to remove the blocking and as a result the whole pipe must be replaced.
Several methods of actively heating the pipe have been described in the art. These methods can be categorized in two groups, namely a group using flowing of hot fluids in selected spaces within the pipe wall and a group using electric heating.
EP 485 220 discloses an electric heating system for subsea flexible pipelines which includes the provision of an electric unit consisting of a controlled rectifying unit, which is the source of current, an electric cable positioned in parallel with the flexible pipeline for the return of the current, and two terminal connectors which electrically isolate the double-reinforced crossed armoring, the electric current being conducted by the tensile armoring or the carcass and returning by an electric cable installed outside the flexible pipeline.
U.S. Pat. No. 7,123,826 discloses a pipe comprising a tubular member formed of a plastic material, and a plurality of electrical current conductive materials dispersed in the plastic material for increasing the electrical conductivity of the tubular layer, so that when electrical power is supplied to the conductor, the current flows through the materials to heat the pipe and the fluids.
US 2012/0217000 discloses a system for electrical heating of risers or pipes which has at least two concentric layers of metal wires adapted to be used for low-voltage direct electric heating (LV-DEH), each pair being provided to heat a specific segment of a riser or a pipe. The system can be used for both the pipeline and the riser up to a top site structure.
In principle the prior art systems provide suitable methods of heating the pipeline. However, there is still a need for an unbonded flexible pipe and an offshore system with such a pipe which provide both a good protection against blocking of the pipe in the event of a temporary production stop while simultaneously being simple and safe.